Elasticity of Anhydrous Phase B Under Mantle Conditions: Implications for the Deep X‐Discontinuity in the Subduction Zones

Abstract Anhydrous phase B (anh‐B) is a dense magnesium silicate with the composition Mg14Si5O24 and space group Pmcb. In magnesium‐rich environments, forsterite reacts with periclase to form anh‐B, and the formation of anh‐B was proposed as a plausible mechanism for the origin of the X‐discontinuity. However, the elastic properties of anh‐B, which are critical for evaluating the seismic features associated with its formation, have not been determined. In this study, we investigated the elasticity of anh‐B at high pressure and temperature via first‐principles calculations. Combining with the elasticity of other minerals, we determined the contrasts caused by the formation of anh‐B: ∼3%, ∼7%, and ∼10% jumps for density, VP, and VS, respectively. The 2%–8% impedance contrasts of the X‐discontinuity can be explained by the formation of 15–60 vol% anh‐B, which requires 3–12 vol% MgO as a reactant.